to POTABLE WATERS (6) FREE AND SEMI-COMBINED CARBON DIOXIDE. To 200 c.c. of the water are added 25 c.c. of N/io-baryta solution and 2 c.c. of highly con- centrated calcium chloride solution. The mixture is left for some hours in a tightly closed vessel; 100 c.c. of the liquid are then drawn off and the excess of baryta present determined with decinormal hydrochloric acid and phenolphthalein. The number of c.c. of acid used is multiplied by 2-37 since the total volume of the liquid was 227 c.c. ; the product is subtracted from the number of c.c. of acid necessary to neutralise. 25 c.c. of the baryta, and the difference, being equivalent to the baryta precipitated by the fret; and semi-combined carbon dioxide, when multiplied by 0-0022 gives the free and semi-combined carbon dioxide in 200 c.c. of the water. 3. Chlorine (Volhard's method}.—To 50 or looc.c. of the water, acidi- fied with nitric acid, is added more than sufficient decinonual, silver nitrate solution to precipitate all the chlorine (usually 5 or 10 c.c. suffice) ; the liquid is shaken, allowed to settle, and filtered, both the precipitating vessel and the filter being well washed.1 To the filtered liquid are added a few drops of ferric alum solution and a little nitric acid, the excess of silver being then determined by titration with N/io ammonium thioeyanate solution until a pinkish-yellow coloration appears. The difference between the volume of silver nitrate taken and that of the thiocyanate required to precipitate the excess of silver, multiplied by O'0o355, gives the. amount of chlorine in grams in the volume of water taken. 4. Sulphuric Acid.—200 c.c. or more (up to I litre) of the water, according as it is rich or poor in sulphates, are acidified with hydrochloric acid and evaporated to small volume (100-150 c.c.), theliquid being then treated with barium chloride, heated, allowed to deposit, and filtered ; the precipitated barium sulphate is washed with water, dried, calcined (the filter paper being burned separately), and weighed : i part of ,Bu,»SO4 ~ . 0*3433 part S03. 5. Nitric Acid.—One litre of the water is evaporated to a few c.c. and the nitric acid then determined by one of the ordinary methods (sec Fertilisers : Determination of Nitric Acid). 6. Phosphoric Acid.—One litre or more of the water is evaporated to a small volume, in which the phosphoric, acid is estimated by precipita- tion as ammonium phosphomolybdate (SM Fertilisers : 1 )etermination of the total Phosphoric Acid). 7. Silica.—One or more litres of the water, according to the. amount of the fixed residue, are evaporated to dryness in a. platinum dish, the residue being dried at 105°, treated with hydrochloric acid, evaporated again and dried at 105°. It is then treated with hydrochloric, acid and water and the liquid filtered, the silica being washed, dried, calcined, and weighed as Si02. 8. Iron and Aluminium.—The filtrate from the preceding operation is treated with a little ammonium chloride and a slight excess of ammonia and heated; if a precipitate is formed, this is filtered off, washed, dried, calcined, and weighed; it represents Fe203 + Ala()3. 1 To save time, after the silver nitrate is added the liquid is made up to a known volume and filtered, an aliquot part being taken for the titration of the excess of silver. ~, (.